Typical methods of monitoring and analyzing single wavelength, single channel optical fiber links do not function with wavelength division multiplexed (WDM) optical networks since each WDM optical fiber can contain multiple channels, each on a different wavelength. Within each WDM optical fiber can be dense groupings of different channels, from as few as two or three up to several hundred. The different densities separate the different channels by wavelengths, called channel spacing. The closer the channel spacing, the more difficult and expensive it is to distinguish the channels in the fiber.
WDM optical networks are typically full duplex, either using two fibers carrying light waves in opposite directions, or a single fiber carrying light waves in both directions. For proper monitoring of WDM optical network, the fiber(s) must be separated into the individual channels so that the data inside the channels can be intercepted and analyzed. Additionally, the two channels corresponding to a single conversation may need to be identified because they generally are not carried on the same wavelength in both directions.